Triggered execution

Run a command when sources change

You can make a command run when certain files change by prefixing the
command with ~. Monitoring is terminated when enter is pressed. This
triggered execution is configured by the watch setting, but typically
the basic settings watchSources and pollInterval are modified as
described in later sections.

The original use-case for triggered execution was continuous
compilation:

> ~ test:compile
> ~ compile

You can use the triggered execution feature to run any command or task,
however. The following will poll for changes to your source code (main
or test) and run testOnly for the specified test.

> ~ testOnly example.TestA

Run multiple commands when sources change

The command passed to ~ may be any command string, so multiple
commands may be run by separating them with a semicolon. For example,

> ~ ;a ;b

This runs a and then b when sources change.

Configure the sources that are checked for changes

watchSources defines the files for a single project that are
monitored for changes. By default, a project watches resources and
Scala and Java sources.

watchTransitiveSources then combines the watchSources for the
current project and all execution and classpath dependencies (see
.scala build definition for details on inter-project
dependencies).

To add the file demo/example.txt to the files to watch,

watchSources += baseDirectory.value / "demo" / "examples.txt"

Set the time interval between checks for changes to sources

pollInterval selects the interval between polling for changes in
milliseconds. The default value is 500 ms. To change it to 1 s,